With each technology node, that is, about every three years, there is a doubling of the quantity of structures on a component surface of the same size. Therefore, the image-generating methods for producing structures on masks and methods for direct structuring of wafers require increasingly longer writing times. Another reason for reduced productivity in high-resolution electron beam writers in high-end mask fabrication is the increasing degree of pre-distortion of the mask structure (optical proximity correction—OPC) to improve the structure resolution of high-productivity scanner objectives whose resolution, as is well-known, is diffraction-limited.
The demand in the semiconductor industry for lowering costs by reducing writing times in high-end mask fabrication and in direct exposure of wafers cannot be met by currently available single-beam writing technologies.
For this reason, alternative multibeam concepts are being adopted to an increasing extent. Multi-shape beam lithography concepts promise an appreciable increase in throughput especially for very high integration levels (<65 nm technology). The concept is based on the idea of simultaneously providing a plurality of particle beams whose shape and size can be adjusted and whose position on the substrate can be controlled. Two main methods for increasing throughput in particle beam lithography systems are known from the prior art.
On one side are the solutions for multibeam systems which work closely in parallel and use large arrays (104-107 beams) of finely focused particle beam bundles of fixed shape and size (electron beam pixels) which are guided substantially collectively over the substrate to be exposed (stage movement and deflection systems) and which are timed to be switched on and off corresponding to the pattern to be exposed. This pixel concept is represented, e.g., by MAPPER (see C. Klein et al., “Projection maskless lithography (PML2): proof-of-concept setup and first experimental results”, Proceedings SPIE Advanced Lithography 2008, vol. 6921-93), and PML2 (see E. Slot et al., “MAPPER: high throughput maskless lithography”, Proceedings SPIE Advanced Lithography 2008, vol. 6921-92). The disadvantages of these concepts are the high complexity of the beam modulators (thousands to hundreds of thousands of deflection systems/lenses) and the high data transfer rates owing to the fact that the circuit layout must be broken down into individual pixels without losses so that any hierarchy or compression is lost.
The second group of solutions is based on variably shaped beams (also known as shaped probes) which are used to expose desired structures in a variable manner by projecting beam cross sections of different area on a substrate (VSB—variable shaped beam).
U.S. Pat. No. 6,703,629 B2 discloses a fairly complex character projection (CP) method in which different masks are imaged one above the other in two planes, and possibly deflected by a deflection system located therebetween, in such a way that typical recurring beam patterns are formed and are then reduced and used for exposure. The drawbacks of this method consist in the fixed choice of character aperture geometry once it has been produced. Another level of technology requiring different conductive path distances or CP dimensions requires a new pair of character apertures. Another disadvantage of this exposure method consists in that, for reasons inherent to the principle, the current density within a character is constant. Accordingly, a correction of the proximity effect depending on the exposure environment, particularly for a large character, is difficult to accomplish, which limits the usefulness and quality of the generated patterns.
It is known from U.S. Pat. No. 7,005,658 B2 to generate an array of particle beams by means of an aperture plate which is illuminated in parallel in that a shared radiation source is collimated by means of a condenser lens. All of the partial beams are corrected individually by correction lenses and deflection systems in such a way that the field distortion and field curvature occurring in the reduction system disappear. The array of spot beams generated in this way is then guided collectively over the substrate to be exposed, and the partial beams are switched on and off (blanking array) at the proper times corresponding to the desired pattern.
A disadvantage in this method is the large amount of pixel data required for exposing a given pattern. Further, a large number of complicated electrostatic correction elements such as lens arrays and deflection arrays are required for controlling the positions and focus planes of all of the partial beams in the target. Another disadvantage in this method is that the individual beams in the target plane have the same size and are located on a fixed position grid. In order to meet current requirements for positioning accuracy of patterns to be exposed (placement 2-5 nm), multiple exposures must be carried out with a slight positional offset (so-called grayscale exposure or gray beaming) which, as is well known, results in a deterioration of structure edges and reduces productivity.
U.S. Pat. No. 5,981,962 A and U.S. Pat. No. 6,175,122 B1 describe an electron beam lithography system as a distributed arrangement of multiple variable shaped beams working in parallel. The concept, which is conceived as a compact miniature system, uses two pinhole diaphragms per electron-optical system which are imaged on one another and a deflection system arranged therebetween for controlling the beam cross section. An external, uniform magnetic field provides for the focused imaging of the diaphragm planes on one another and on the target. It is suggested that the position deflection in the target is carried out by moving the substrate stage in one direction and by collective, line-by-line electrostatic deflection in orthogonal direction. Every miniaturized VSB system (variable shaped beam system) is supplied by separate electron sources (emitter arrays).
A disadvantage in this method is the 1:1 imaging of the beam-shaping diaphragms in the target plane. The required edge roughness of lithographic structures is presently in the range of a few nanometers for advanced technologies. Accordingly, the quality of the diaphragms to be used would have to be even better, which appears very difficult in terms of technology, particularly with respect to the generation of such small corner radii. Contamination effects at the diaphragm edges in practical operation are likewise effective in a ratio of 1:1 during exposure and therefore limit the quality of the patterns and the life of the diaphragms. Further, the resources required for providing an entire array of radiation sources and for monitoring them individually is disadvantageous. A very high mechanical accuracy and high uniformity of the magnetic field is required in order to maintain the focusing condition simultaneously for all beam bundles, which can only be achieved at great expense.
Further, it remains unclear how collective focusing is to be carried out when the target (e.g., machined wafer) has inevitable residual unevenness. Finally, the collective deflection of all of the beams in the target plane presents a severe limitation on the quantity of beams that can be used simultaneously for exposure or defines a fixed grid for pattern generation. The fixed grid of the beam bundles must constantly be aligned with the necessary reference positions of the patterns to be exposed. In the case of very precise draft grids of the patterns, this substantially limits productivity and/or flexibility of the exposure device.
U.S. Pat. No. 6,614,035 B2 describes a multibeam system whose operation is very similar to that of the known VSB system (single beam). In order to separate a plurality of individually controllable beams in the area of the two beam-shaping diaphragm planes, it is suggested that a diaphragm is arranged at that location which subdivides the conventional illuminated area (diaphragm aperture) by inserting struts into a plurality of openings. Every beam bundle formed in this manner receives four individual deflection systems which are arranged in two portions of the electron-optical column and carry out two independent functions. The upper, first deflection system stage serves to individually adjust the beam cross section and the second defecting system stage serves to adjust the distance between adjacent beam bundles within certain limits. The subsequent reduction and positioning of the array of beam bundles in the lower imaging portion is carried out collectively and in exactly the same way as in a VSB system.
This concept has the disadvantage that the deflection of the beam bundles in two orthogonal directions is carried out in only one plane because, in deflection arrangements with such close proximity of the beams, simultaneous deflection in two orthogonal directions leads to large deflection errors which impair the edge quality and uniformity of the illumination. This also applies to deflection systems for individual control of the mutual beam distance in an array of partial beams that are to be controlled individually. Further, there are no concrete proposals for preventing crosstalk between the adjacent deflection systems which are arranged within a very confined space in proximity to the beam for every beam of the multibeam system.
Another weakness in the concept is the use of a plurality of small variable beam cross sections of comparable size for the exposure process of an entire design layout. Typically, the layout to be exposed does not contain exclusively very small structures even at an advanced level of integration, but rather also larger structures. The gain in productivity which is achieved when exposing many small structures by means of an array of 4 to 16 beams working in parallel can be partially negated when the layout contains a series of relatively large patterns with unfavorable spacing.